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Tue, 5 Jul 2022 12:27:13 +0000 (UTC) X-FDA: 79652971146.21.1B51DDF Received: from out30-133.freemail.mail.aliyun.com (out30-133.freemail.mail.aliyun.com [115.124.30.133]) by imf20.hostedemail.com (Postfix) with ESMTP id 36CEE1C0007 for ; Tue, 5 Jul 2022 12:27:09 +0000 (UTC) X-Alimail-AntiSpam:AC=PASS;BC=-1|-1;BR=01201311R181e4;CH=green;DM=||false|;DS=||;FP=0|-1|-1|-1|0|-1|-1|-1;HT=ay29a033018046051;MF=guanghuifeng@linux.alibaba.com;NM=1;PH=DS;RN=21;SR=0;TI=SMTPD_---0VISH683_1657024024; Received: from 30.225.28.170(mailfrom:guanghuifeng@linux.alibaba.com fp:SMTPD_---0VISH683_1657024024) by smtp.aliyun-inc.com; Tue, 05 Jul 2022 20:27:05 +0800 Message-ID: Date: Tue, 5 Jul 2022 20:27:03 +0800 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:91.0) Gecko/20100101 Thunderbird/91.11.0 Subject: Re: [PATCH v4] arm64: mm: fix linear mem mapping access performance degradation To: Will Deacon Cc: Ard Biesheuvel , baolin.wang@linux.alibaba.com, catalin.marinas@arm.com, akpm@linux-foundation.org, david@redhat.com, jianyong.wu@arm.com, james.morse@arm.com, quic_qiancai@quicinc.com, christophe.leroy@csgroup.eu, jonathan@marek.ca, mark.rutland@arm.com, thunder.leizhen@huawei.com, anshuman.khandual@arm.com, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, rppt@kernel.org, geert+renesas@glider.be, linux-mm@kvack.org, yaohongbo@linux.alibaba.com, alikernel-developer@linux.alibaba.com References: <20220704111402.GA31553@willie-the-truck> <4accaeda-572f-f72d-5067-2d0999e4d00a@linux.alibaba.com> <20220704131516.GC31684@willie-the-truck> <2ae1cae0-ee26-aa59-7ed9-231d67194dce@linux.alibaba.com> <20220704142313.GE31684@willie-the-truck> <6977c692-78ca-5a67-773e-0389c85f2650@linux.alibaba.com> <20220704163815.GA32177@willie-the-truck> <20220705095231.GB552@willie-the-truck> <5d044fdd-a61a-d60f-d294-89e17de37712@linux.alibaba.com> <20220705121115.GB1012@willie-the-truck> From: "guanghui.fgh" In-Reply-To: <20220705121115.GB1012@willie-the-truck> Content-Type: text/plain; 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cv=none; b=CHDZPspWGsZuIC7cjwCfB6EXEugzHQNhbViyD9wQ4lKqWoYwfbzu+GpnKHzTEVYxsNVavI yqrGrAQCCNrapRxsY9A3FWizqm15GMANSY49j1TM56DSf/kwJ1slvLccCn95vkdW//RQ3f pTIncJFdep+C4qzzZ5oiQOiHCxusfTI= X-Stat-Signature: s15m3xkmfr6a18djqmtpnwomciz5g3ge X-Rspamd-Queue-Id: 36CEE1C0007 Authentication-Results: imf20.hostedemail.com; dkim=none; dmarc=pass (policy=none) header.from=alibaba.com; spf=pass (imf20.hostedemail.com: domain of guanghuifeng@linux.alibaba.com designates 115.124.30.133 as permitted sender) smtp.mailfrom=guanghuifeng@linux.alibaba.com X-Rspamd-Server: rspam03 X-Rspam-User: X-HE-Tag: 1657024029-776896 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: 在 2022/7/5 20:11, Will Deacon 写道: > On Tue, Jul 05, 2022 at 08:07:07PM +0800, guanghui.fgh wrote: >> >> >> 在 2022/7/5 17:52, Will Deacon 写道: >>> On Mon, Jul 04, 2022 at 07:09:23PM +0200, Ard Biesheuvel wrote: >>>> On Mon, 4 Jul 2022 at 18:38, Will Deacon wrote: >>>>> >>>>> On Mon, Jul 04, 2022 at 10:34:07PM +0800, guanghui.fgh wrote: >>>>>> Thanks. >>>>>> >>>>>> 在 2022/7/4 22:23, Will Deacon 写道: >>>>>>> On Mon, Jul 04, 2022 at 10:11:27PM +0800, guanghui.fgh wrote: >>>> ... >>>>>>>> Namely, it's need to use non block/section mapping for crashkernel mem >>>>>>>> before shringking. >>>>>>> >>>>>>> Well, yes, but we can change arch_kexec_[un]protect_crashkres() not to do >>>>>>> that if we're leaving the thing mapped, no? >>>>>>> >>>>>> I think we should use arch_kexec_[un]protect_crashkres for crashkernel mem. >>>>>> >>>>>> Because when invalid crashkernel mem pagetable, there is no chance to rd/wr >>>>>> the crashkernel mem by mistake. >>>>>> >>>>>> If we don't use arch_kexec_[un]protect_crashkres to invalid crashkernel mem >>>>>> pagetable, there maybe some write operations to these mem by mistake which >>>>>> may cause crashkernel boot error and vmcore saving error. >>>>> >>>>> I don't really buy this line of reasoning. The entire main kernel is >>>>> writable, so why do we care about protecting the crashkernel so much? The >>>>> _code_ to launch the crash kernel is writable! If you care about preventing >>>>> writes to memory which should not be writable, then you should use >>>>> rodata=full. >>>>> >>>> >>>> This is not entirely true - the core kernel text and rodata are >>>> remapped r/o in the linear map, whereas all module code and rodata are >>>> left writable when rodata != full. >>> >>> Yes, sorry, you're quite right. The kernel text is only writable if >>> rodata=off. >>> >>> But I still think it makes sense to protect the crashkernel only if >>> rodata=full (which is the default on arm64) as this allows us to rely >>> on page mappings and I think fits well with what we do for modules. >>> >>>> But the conclusion is the same, imo: if you can't be bothered to >>>> protect a good chunk of the code and rodata that the kernel relies on, >>>> why should the crashkernel be treated any differently? >>> >>> Thanks. >>> >>> Will >> Thanks. >> >> 1.The rodata full is harm to the performance and has been disabled in-house. >> >> 2.When using crashkernel with rodata non full, the kernel also will use non >> block/section mapping which cause high d-TLB miss and degrade performance >> greatly. >> This patch fix it to use block/section mapping as far as possible. >> >> bool can_set_direct_map(void) >> { >> return rodata_full || debug_pagealloc_enabled(); >> } >> >> map_mem: >> if (can_set_direct_map() || IS_ENABLED(CONFIG_KFENCE)) >> flags |= NO_BLOCK_MAPPINGS | NO_CONT_MAPPINGS; >> >> 3.When rodata full is disabled, crashkernel also need protect(keep >> arch_kexec_[un]protect_crashkres using). >> I think crashkernel should't depend on radata full(Maybe other architecture >> don't support radata full now). > > I think this is going round in circles :/ > > As a first step, can we please leave the crashkernel mapped unless > rodata=full? It should be a much simpler patch to write, review and maintain > and it gives you the performance you want when crashkernel is being used. > > Will Thanks. There is a circle. 1.When the rodata is non full, there will be some error when calling arch_kexec_[un]protect_crashkres(BUG_ON(pud_huge(*pud))) now. It's also need non-block/section mapping for crashkernel mem. 2.In other words, maybe we should change arch_kexec_[un]protect_crashkres to support block/section mapping which can leave crashkernel block/section mapping. But when we shrink the crashkernel mem, we also need to split some block/section mapping(part mem for crashkernel, the left for the normal kernel). As a result, maybe we build crashkernel mem with non-block/section mapping is appropriate(as this patch doing).